'New outbreaks'

In a report released on Monday at its annual meeting in Manila, the WHO said it is "highly likely that the region will continue to report new cases and possibly new outbreaks of Zika".

This is because the carrier of the disease - the Aedes mosquito - is widely found in the region, which also sees a high volume of travel.

There is also still uncertainty about the level of immunity in the regional population, the report added.

"Unfortunately, scientists do not yet have answers to many critical questions [about Zika]," Ms Chan said at the meeting.

Though the virus is known to have existed in Asia for decades, she noted that the first sign of its presence in some countries came from travellers whose Zika infections were only confirmed once they returned home.

"Is this weak surveillance, an indication of population-wide immunity, or proof that the virus has somehow acquired greater epidemic potential?" she asked.

Zika generally has mild effects but is more dangerous for pregnant women as it has been linked to severe birth defects, including the brain deformation microcephaly.